A great amount of labor goes into the construction of a rubber tire. Layer after layer of various rubber compounds, fabrics, cords and steel wires are assembled in molds where they are compressed and heated under pressure to insure that the rubber will flow into the mold patterns and vulcanize to the proper resiliency. The final result is a varying amount of irregularities which differ from tire to tire.
After construction, the tires are mounted on wheels and then dynamically balanced by applying weights to the rim of the wheels, whereupon the balanced tires are mounted on a vehicle. It is a universally known fact that all tire balancing machines operate with the tire/wheel assembly in space, i.e., in an unloaded condition. The tire is never loaded during balancing to simulate the conditions which are encountered when the tire is in use. A tire balanced in conventional fashion is assumed to be geometrically true about its axis of rotation, which assumes that the tire in use will operate at a constant rolling radius. This is very seldom the case. Three types of asymmetrical conditions are ever present: (1) radial variations (geometrical irregularties), (2) asymmetrical structural distribution (rigidity variations), and (3) asymmetrical distribution of the mass (unbalance).
Tires are supported by the roadway in much the same manner as a track supports a railroad wheel, but the railroad wheel is made of steel and is inflexible for practical purposes. The perimeter of the railroad wheel is machined so as to be concentric with its axis of rotation and therefore contacts the rail at a constant loaded radius. Tires, on the other hand, are pneumatic annular envelopes intended to reduce shock while flexing at the footprint or tire patch, i.e., the area of the tire in contact with the roadway. A tire by reason of the nature of its construction is not symmetrical and therefore the balancing techniques utilized to balance a railroad wheel or an armature or a crankshaft are ineffective to properly balance a rubber tire. The vibratory reactions occurring within the tire, which depend upon the speed and load at which the tire is rotated, are particularly present on smooth roads due to the lack of peripheral uniformity of the tire. It is these vibratory centrifugal reactions which must be overcome if the tire is to operate at a quasi constant rolling radius.